Light-emitting device and manufacturing method thereof

ABSTRACT

A light-emitting device, includes a substrate structure, including a base portion having a surface and a plurality of protrusions regularly formed on the base portion; a buffer layer covering the plurality of protrusions and the surface; and III-V compound semiconductor layers formed on the buffer layer; wherein one of the plurality of protrusions includes a first portion and a second portion formed on the first portion and the first portion is integrated with the base portion; and wherein the base portion includes a first material and the first portion includes the first material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 16/397,775, filed on Apr. 29, 2019, which claims priority to the benefit of Taiwan Application Serial Number 107114617 filed on Apr. 30, 2018, and the entire contents of which are hereby incorporated by reference herein in its entirety.

BACKGROUND Technical Field

The present disclosure relates to a light-emitting device, more specifically, to a light-emitting device including a substrate structure.

Description of the Related Art

The light-emitting diodes (LEDs) have been widely used in solid-state lighting. The LEDs have the characteristics of low power consumption and long life time compared with conventional incandescent lamp and fluorescent lamp. Therefore, the LEDs have gradually replaced conventional light source and are applied to various fields, such as traffic lamps, backlight modules, streetlights, medical equipment and the like.

SUMMARY

A light-emitting device, includes a substrate structure, including a base portion having a surface and a plurality of protrusions regularly formed on the base portion; a buffer layer covering the plurality of protrusions and the surface; and III-V compound semiconductor layers formed on the buffer layer; wherein one of the plurality of protrusions includes a first portion and a second portion formed on the first portion and the first portion is integrated with the base portion; and wherein the base portion includes a first material and the first portion includes the first material.

A method of manufacturing a light-emitting device, including: providing a substrate structure including a base portion and wherein the base portion includes a surface; performing a patterning step to form a plurality of protrusions, wherein the plurality of protrusions are arranged on the surface of the base portion; forming a buffer layer on the surface of the base portion by physical vapor deposition, wherein the buffer layer covers the protrusions; and forming III-V compound semiconductor layers on the buffer layer; wherein one of the plurality of protrusions has a height not greater than 1.5 μm; and wherein the light-emitting device has a full width at half maximum (FWHM) of smaller than 250 arcsec in accordance with a (102) XRD rocking curve.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a light-emitting device in accordance with a first embodiment of the present application.

FIGS. 2A and 2B show a light-emitting device in accordance with a second embodiment of the present application.

FIGS. 3A and 3B show a light-emitting device in accordance with a third embodiment of the present application.

FIGS. 4A and 4B show top views of a substrate structure of the light-emitting device in accordance with different embodiments of the present application, respectively.

FIGS. 5A and 5B show cross-sectional views of a protrusion of the light-emitting device in accordance with different embodiments of the present application, respectively.

DETAILED DESCRIPTION OF THE EMBODIMENTS

To better and concisely explain the disclosure, the same name or the same reference number given or appeared in different paragraphs or figures along the specification should has the same or equivalent meanings while it is once defined anywhere of the disclosure.

FIGS. 1A and 1B show a light-emitting device 100 in accordance with a first embodiment of the present application. FIG. 1A shows a cross-sectional view of the light-emitting device 100. As shown in FIG. 1A, the light-emitting device 100 includes a substrate structure 102, a buffer layer 104 formed on the substrate structure 102, a first semiconductor layer 106 formed on the buffer layer 104, a light-emitting structure 108 formed on the first semiconductor layer 106 and a second semiconductor layer 110 formed on the light emitting structure 108. The first semiconductor layer 106, the light emitting structure 108 and the second semiconductor layer 110 include III-V compound semiconductor, such as aluminum gallium indium nitride (In_(x)Al_(y)Ga_(1-x-y)N, 0≤x≤1, 0≤y≤1). The light-emitting structure 108 can be a single heterostructure (SH), a double heterostructure (DH), a double-side double heterostructure (DDH), or a multi-quantum well, MQW). As shown in FIG. 1A, a direction T1 indicates the growth direction of the light-emitting structure 108. The light-emitting structure 108 generates radiation. In this embodiment, the radiation includes light. The light can be visible or invisible. The light has a dominant wavelength which can be between 250 nm and 500 nm. In this embodiment, the light-emitting device 100 further includes a first electrode 120 and a second electrode 130. The first electrode 120 is formed on the first semiconductor layer 106 and electrically connects to the first semiconductor layer 106. The second electrode 130 is formed on the second semiconductor layer 106 and electrically connects to the second semiconductor layer 106.

Referring to FIG. 1B and FIG. 1A, FIG. 1B is a partial enlarged view of the substrate structure 102 of FIG. 1A. The substrate structure 102 includes a base portion 102 b and a plurality of protrusions 102 c. The base portion 102 b has a surface 102 a. In one embodiment, the thickness of the base portion 102 b is not less than 100 μm. In one embodiment, the thickness of the base portion 102 b is not more than 300 μm. The plurality of protrusions 102 c is arranged in a two-dimensional array on the surface 102 a of the base portion 102 b, and the arrangement of the plurality of protrusions 102 c can be regular or irregular. Each of the protrusions 102 c includes a first material and the base portion 102 b includes a second material. The refractive index of the first material is smaller than the refractive index of the second material. Specifically, the refractive index of the first material of the protrusion 102 c at the dominant wavelength is smaller than the refractive index of the second material of the base portion 102 b at the dominant wavelength. In one embodiment, at the dominant wavelength, the difference between the refractive index of the first material of the protrusion 102 c and the refractive index of the second material of the base portion 102 b is greater than 0.1; in another embodiment, the difference is greater than 0.15; and still in another embodiment, the difference is between 0.15 and 0.4 (both inclusive). The material of the base portion 102 b can be sapphire, and the surface 102 a is c-plane which is suitable for epitaxial growth. The material of each of the protrusions 102 c can be silicon dioxide (SiO₂). The three-dimensional shape of the protrusion 102 c includes a dome, a cone, or a pyramid. The cone includes a truncated cone and the pyramid includes a polygonal pyramid or a truncated pyramid. In the embodiment, the three-dimensional shape of the protrusion 102 c is a cone, wherein the protrusion 102 c is substantially triangular in the cross-sectional view of the light-emitting device. The buffer layer 104 is conformally formed on the plurality of protrusion 102 c and the surface 102 a. Specifically, the buffer layer 104 has a top surface 1041 opposite to the base portion 102 b. The top surface 1041 includes a first portion 1042 and a second portion 1043 connected to the first portion 1042. The first portion 1042 covers the surface 102 a. The second portion 1043 covers the plurality of protrusions 102 c. In the cross-sectional view of the light-emitting device 100, a recess 111 is defined between the first portion 1042 and the second portion 1043. In one embodiment, the buffer layer 104 comprises aluminum nitride (AlN). In one embodiment, the thickness of the buffer layer 104 is greater than 5 nm; in another embodiment, the thickness is not more than 50 nm; and still in another embodiment, the thickness is between 10 nm and 30 nm (both inclusive). If the thickness of the buffer layer 104 is less than 5 nm, the defect density of the epitaxial layers (e.g. the first semiconductor layer 106) subsequently grown thereon becomes high and the epitaxial quality of the light-emitting device is disrupted. If the thickness of the buffer layer 104 is more than 50 nm, for example, an AlN buffer layer with a thickness of more than 50 nm, the dominant wavelengths of the plurality of light-emitting devices epitaxially grown on the same wafer are inconsistent with each other. As shown in FIG. 1B, an included angle θ is between the surface 102 a and the side surface of one protrusion 102 c or each of the protrusions 102 c. In one embodiment, θ is not greater than 65 degrees; in another embodiment, θ is not more than 55 degrees; and still in another embodiment, θ is between 30 and 65 degrees (both inclusive). In one embodiment, two included angles θ are between the surface 102 a and the side surface of one protrusion 102 c or each of the protrusions 102 c and θ are not greater than 65 degrees; in another embodiment, θ are not greater than 55 degrees; and still in another embodiment, θ are between 30 and 55 degrees (both inclusive). In one embodiment, the two included angles θ of one protrusion 102 c or the two included angles θ of each protrusion 102 c have the same degrees or different degrees. Each protrusion 102 c has a height H and a bottom width W. In the present embodiment, the height H is not more than 1.5 and in another embodiment, the height H is between 0.5 μm and 1.5 μm (both inclusive). The bottom width W is not less than 1 and in another embodiment, the bottom width W is between 1 μm and 3 μm (both inclusive). In one embodiment, the ratio of the height H to the bottom width W is greater than 0 and not more than 0.5. In another embodiment, the ratio of the height H to the bottom width W is between 0.4 and 0.5 (both inclusive). As shown in the figures, the arrangement of the protrusions 102 c has a period P. In one embodiment, the protrusion 102 c has a vertex, and the vertex is the part of the protrusion 102 c which is closest to the light-emitting structure 108 along the direction T1. The period P is defined as the distance between the vertices of two adjacent protrusions 102 c. In this embodiment, the cross-section of the protrusion 102 c is substantially triangular, and the period P is between 1 μm and 3 μm (both inclusive). In one embodiment, the height H=1.2 μm±10%; the bottom width W=2.6 μm±10%; the period P=3.0 μm±10%. In another embodiment, the height H=0.9 μm±10%; the width W=1.6 μm±10%; the period P=1.8 μm±10%. In another embodiment, H=1 μm±10%; W=1.5 μm 10%; P=1.8 μm±10%. In another embodiment, the height H=1.2 μm±10%, the width W=2.6 μm±10%; and the period P=3.0 μm±10%. In one embodiment, x-ray diffraction (XRD) analysis is used to investigate the material characteristics of the epitaxy layers of the light-emitting device 100. The light-emitting device 100 has a FWHM (full width at half maximum) value of smaller than 250 arcsec in accordance with a (102) XRD rocking curve. In one embodiment, the FWHM value is not smaller than 100 arcsec. Light emitted from the light-emitting structure 108 is reflected and/or refracted due to the plurality of protrusions 102 c formed on the surface 102 a of the substrate structure 102 so that the light emitting efficiency of the light-emitting device 100 is improved. Moreover, in this embodiment, because of the substrate structure 102 coupled with the buffer layer 104, the quality of the semiconductor layers and the light-emitting structure 108 grown thereon is improved.

As shown in FIG. 1A, a method for fabricating a light-emitting device 100 in accordance with an embodiment of the present application includes: providing a substrate structure 202 including the base portion 202 b and the plurality of protrusions 202 c formed thereon, wherein the base portion 202 b has a surface 202 a; and performing a patterning step to form the plurality of protrusions 202 c. The patterning step includes forming a precursor layer (not shown) on the surface 102 a by, for example, physical vapor deposition (PVD), and then removing a portion of the precursor layer. The method of removing the portion of the precursor layer includes dry etching or wet etching. The portion of the precursor layer is removed and then the other portion of the precursor layer kept on the surface 202 a forms the plurality of protrusions 202 c separated from each other. In the embodiment, the protrusion is substantially triangular in the cross-sectional view. The plurality of protrusions 202 c is arranged in a two-dimensional array on the surface 202 a of the base portion 202 b, and the arrangement of the plurality of protrusions 202 c can be regular or irregular. The method for fabricating the light-emitting device 100 in accordance with an embodiment of the present application further includes forming the buffer layer 104 on the surface 102 a of the base portion 202 b and covering the protrusions 202 c. The buffer layer 204 comprises aluminum nitride (AlN). The method of forming a buffer layer 104 includes physical vapor deposition (PVD). The method of fabricating the light-emitting device 100 further includes forming the first semiconductor layer 206, the light-emitting structure 208 and the second semiconductor layer 210 by epitaxial growth such as metal organic chemical vapor deposition (MOCVD). The method of epitaxial growth includes, but is not limited to, MOCVD, hydride vapor phase epitaxial (HVPE), or liquid-phase epitaxy (LPE).

As shown in the embodiment of FIG. 1A, the light-emitting device 100 further includes a cap layer 140 between the buffer layer 104 and the first semiconductor layer 106. The cap layer 140 has a thickness T that is greater than the thickness of the buffer layer 104. The cap layer 140 comprises III-V compound semiconductor having an energy gap smaller than that of the material of the buffer layer 104. In an embodiment, the cap layer 140 comprises gallium nitride (GaN). Specifically, the cap layer 140 covers the buffer layer 104, and a portion of the cap layer 140 is located in the recess 111. The cap layer 140 includes a top surface 1401 opposite to the buffer layer 104, and the thickness T of the cap layer 140 refers to the shortest distance between the first portion 1042 of the top surface 1041 and the top surface 1401. In one embodiment, the cap layer 140 has a thickness T greater than 1 μm; in another embodiment, T is not more than 3.5 μm; and still in another embodiment, T is between 1 μm and 2 μm. In an embodiment, the cap layer 140 does not include dopants that are intentionally doped. Specifically, the doping concentration of the cap layer 140 is not more than 5×10¹⁷/cm³, and in one embodiment, not more than 1×10¹⁷/cm³. In this embodiment, since the plurality of protrusions 102 c is formed on the surface 102 a of the substrate structure 102 and the height of the protrusions 102 c is not more than 1.5 μm, the cap layer 140 of the light-emitting device 100 is thinner than that of the conventional light-emitting device, yet the light-emitting device 100 has the same performance as the conventional light-emitting device. Therefore, the light-emitting device 100 in the embodiment has an advantage of being small in size.

FIGS. 2A and 2B show a light-emitting device 200 in accordance with another embodiment of the present application. As shown in FIG. 2A, the light-emitting device 200 includes a substrate structure 202, a buffer layer 204 formed on the substrate structure 202, a cap layer 240 formed on the buffer layer 201, a first semiconductor layer 206 formed on the cap layer 240, a light emitting structure 208 formed on the first semiconductor layer 206 and a second semiconductor layer 210 formed on the light emitting structure 208. The first semiconductor layer 206, the light emitting structure 208 and the second semiconductor layer 210 include III-V compound semiconductor, such as aluminum gallium indium nitride (In_(x)Al_(y)Ga_(1-x-y)N, The light emitting structure 208 includes a single heterostructure (SH), a double heterostructure (DH), a double-side double heterostructure (DDH), or a multi-quantum well structure, MQW). As shown in FIG. 2A, the direction T1 indicates the growth direction of the light emitting structure 208. The light emitting structure 208 generates radiation. In this embodiment, the radiation includes light. The light can be visible or invisible. The light has a dominant wavelength between 250 nm and 500 nm. In this embodiment, the light-emitting device 200 includes a first electrode 220 and a second electrode 230. The first electrode 220 is formed on the first semiconductor layer 206 and electrically connects to the first semiconductor layer 206. The second electrode 230 is formed on the second semiconductor layer 206 and electrically connects to the second semiconductor layer 206.

Referring to FIG. 2B and FIG. 2A, FIG. 2B is a partially enlarged view of the substrate structure 202 of FIG. 2A. The substrate structure 202 includes a base portion 202 b and a plurality of protrusions 202 c. The base portion 202 b has a surface 202 a. In one embodiment, the thickness of the base portion 202 b is not less than 100 μm; in another embodiment, the thickness of the base portion 202 b is not more than 300 μm. The plurality of protrusions 202 c is arranged in a two-dimensional array on the surface 202 a of the base portion 202 b, and the arrangement of the plurality of protrusions 202 c can be regular or irregular. In the embodiment, each protrusion 202 c is integrated with the base portion 202 b. Specifically, each protrusion 202 c and the base portion 202 b have the same material such as sapphire, and the surface 202 a is c-plane which is suitable for epitaxial growth. The three-dimensional shape of the protrusion 202 c includes a dome, a cone, or a pyramid. The cone includes a truncated cone and the pyramid includes a polygonal pyramid or a truncated pyramid. In the embodiment, the three-dimensional shape of the protrusion 202 c is a cone and the protrusion 202 c is substantially triangular in the cross-sectional view of the light-emitting device. The buffer layer 204 is conformally formed on the plurality of protrusion 202 c and the surface 202 a. Specifically, the buffer layer 204 has a top surface 2041 opposite to the base portion 202 b. The top surface 2041 includes a first portion 2042 and a second portion 2043 connected to the first portion 2042. The first portion 2042 covers the surface 202 a. The second portion 2043 covers the plurality of protrusions 202 c. In the cross-sectional view of the light-emitting device, a recess 211 is formed between the first portion 2042 and the second portion 2043. In one embodiment, the buffer layer 204 comprises aluminum nitride (AlN). The thickness of the buffer layer 204 is greater than 5 nm; in another embodiment, the thickness of the buffer layer 204 is not more than 50 nm; and still in another embodiment, the thickness of the buffer layer 204 is between 10 nm and 30 nm (both inclusive). If the thickness of the buffer layer 204 is less than 5 nm, the defect density of the epitaxial layers subsequently grown thereon (e.g. the first semiconductor layer 206) becomes high and the epitaxial quality of the light-emitting device is disrupted. If the thickness of the buffer layer 104 is more than 50 nm, for example, an AlN buffer layer with a thickness of more than 50 nm, the dominant wavelengths of the plurality of light-emitting devices epitaxially grown on the same wafer are inconsistent with each other. As shown in FIG. 2B, an included angle θ is between the surface 202 a and the side surface of one protrusion 202 c or each of the protrusions 102 c. In one embodiment, θ is not greater than 65 degrees; in another embodiment, θ is not more than 55 degrees; and still in another embodiment, θ is between 30 and 65 degrees (both inclusive). In one embodiment, two included angles θ are between the surface 202 a and the side surface of one protrusion 202 c or each of the protrusions 202 c and θ are not greater than 65 degrees. In one embodiment, θ are not greater than 55 degrees. In another embodiment, θ are between 30 and 55 degrees (both inclusive). In one embodiment, the two included angles θ of one protrusion 202 c or the two included angles θ of each protrusion 202 c have the same degrees or different degrees. Each protrusion 202 c has a height H and a bottom width W. In the present embodiment, the height H is not more than 1.5 μm. In another embodiment, the height H is between 0.5 μm and 1.5 μm (both inclusive). The bottom width W is not less than 1 and in another embodiment, the bottom width W is between 1 μm and 3 μm (both inclusive). In one embodiment, the ratio of the height H to the bottom width W is greater than 0 and not more than 0.5. In another embodiment, the ratio of the height H to the bottom width W is between 0.4 and 0.5 (both inclusive). As shown in the figures, the arrangement of the protrusions 202 c has a period P. In one embodiment, the protrusion 202 c has a vertex, and the vertex is the part of the protrusion 202 c which is closest to the light-emitting structure 208 along the direction T1. The period P is defined as the distance between the vertices of two adjacent protrusions 202 c. In this embodiment, the cross-section of the protrusion 202 c is substantially triangular, and the period P is between 1 μm and 3 μm (both inclusive). In one embodiment, the height H=1.2 μm±10%; the bottom width W=2.6 μm±10%; the period P=3.0 μm±10%. In another embodiment, the height H=0.9 μm±10%; the width W=1.6 μm±10%; the period P=1.8 μm±10%. In another embodiment, H=1 μm±10%; W=1.5 μm 10%; P=1.8 μm±10%. In another embodiment, the height H=1.2 μm±10%, the width W=2.6 μm±10%; and the period P=3.0 μm±10%. In one embodiment, XRD analysis is used to investigate the material characteristics of the epitaxy layers of the light-emitting device 200. The light-emitting device 200 has a FWHM value of smaller than 250 arcsec in accordance with a (102) XRD rocking curve. In one embodiment, the FWHM value is not smaller than 100 arcsec. Light emitted from the light-emitting structure 208 is reflected and/or refracted due to the plurality of protrusions 202 c formed on the surface 202 a of the substrate structure 202 so that the light emitting efficiency of the light-emitting device 200 is improved. Moreover, because of the substrate structure 202 coupled with the buffer layer 204 in accordance with the present embodiment, the quality of the semiconductor layers and the light-emitting structure 208 grown thereon is improved.

As shown in FIG. 2A, a method for fabricating a light-emitting device 200 in accordance with an embodiment of the present application includes: providing a substrate structure 202 including the base portion 202 b and the plurality of protrusions 202 c formed thereon, wherein the base portion 202 b has a surface 202 a. The plurality of protrusions 202 c is arranged in a two-dimensional array on the surface 202 a of the base portion 202 b, and the arrangement of the plurality of protrusions 202 c can be regular or irregular. A buffer layer 204 is formed on the surface 202 a of the base portion 202 b and covers the protrusions 202 c. The buffer layer 204 comprises aluminum nitride (AlN). In one embodiment, the method of fabricating the substrate structure 202 includes providing a substrate (not shown) having a top surface and then performing a patterning step including removing a portion of the substrate from the top surface of the substrate to form the base portion 202 b and the plurality of protrusions 202 c spaced apart with each other on the base portion 202 b. The base portion 202 b has the surface 202 a. The method of removing the portion of substrate includes dry etching or wet etching. In this embodiment, the protrusion 202 c is substantially triangular in the cross-sectional view of the light-emitting device 200. A method of forming a buffer layer 204 includes physical vapor deposition (PVD). The method of fabricating the light-emitting device 200 includes forming the first semiconductor layer 206, the light-emitting structure 208, and the second semiconductor layer 210 by metal organic chemical vapor deposition (MOCVD). The method of performing epitaxial growth includes, but is not limited to, MOCVD, hydride vapor phase epitaxial (HVPE), or liquid-phase epitaxy (LPE).

As shown in FIG. 2A, in the present embodiment, the light-emitting device 200 further includes a cap layer 240 between the buffer layer 204 and the first semiconductor layer 206. The cap layer 240 has a thickness T that is greater than the thickness of the buffer layer 204. The cap layer 240 comprises III-V compound semiconductor having an energy gap smaller than that of the material of the buffer layer 204. In an embodiment, the cap layer 240 includes gallium nitride (GaN). Specifically, the cap layer 240 covers the buffer layer 204, and a portion of the cap layer 240 is located in the recess 211. The cap layer 240 includes a top surface 2401 opposite to the buffer layer 204, and the thickness T of the cap layer 240 refers to the shortest distance between the first portion 2042 and the top surface 2401. In one embodiment, the thickness T of the cap layer 240 is greater than 1 μm. In one embodiment, the thickness T of the cap layer 240 is not greater than 3.5 μm. In one embodiment, the thickness T of the cap layer 240 ranges from 1 μm to 2 μm. In one embodiment, the cap layer 240 includes impurities that are not intentionally doped. Specifically, the impurity concentration of the cap layer 140 is not more than 5×10¹⁷/cm³. In another embodiment, the impurity concentration of the cap layer 140 not more than 1×10¹⁷/cm³. In this embodiment, since the plurality of protrusions 202 c is formed on the surface 202 a of the substrate structure 202 and the height of the protrusions 202 c is not more than 1.5 the cap layer 240 of the light-emitting device 200 is thinner than that of the conventional light-emitting device, yet the light-emitting device 200 has the same performance as the conventional light-emitting device. Therefore, the light-emitting device 200 in the embodiment has an advantage of being small in size.

FIGS. 3A and 3B show another embodiment of a light-emitting device 300 of the present application. As shown in FIG. 3A, the light-emitting device 300 includes a substrate structure 302, a buffer layer 304 formed on the substrate structure 302, a first semiconductor layer 306 formed on the buffer layer 304, a light emitting structure 308 formed on the first semiconductor layer 306 and a second semiconductor layer 310 formed on the light emitting structure 308. The first semiconductor layer 306, the light emitting structure 308, and the second semiconductor layer 310 include III-V compound semiconductor, such as aluminum gallium indium nitride (In_(x)Al_(y)Ga_(1-x-y)N, 0≤x≤1, 0≤y≤1). The light emitting structure 308 includes a single heterostructure (SH), a double heterostructure (DH), a double-side double heterostructure (DDH), or a multi-quantum well structure, MQW). As shown in FIG. 3A, the direction T1 indicates the growth direction of the light emitting structure 308. The light emitting structure 308 generates radiation. In this embodiment, the radiation includes light. The light can be visible or invisible. The light has a dominant wavelength between 250 nm and 500 nm. The light-emitting device 300 further includes a first electrode 320 and a second electrode 330. The first electrode 320 is formed on the first semiconductor layer 306 and electrically connects to the first semiconductor layer 306. The second electrode 330 is formed on the second semiconductor layer 306 and electrically connects to the second semiconductor layer 306.

Referring to FIG. 3B and FIG. 3A, FIG. 3B is a partially enlarged view of the substrate structure 302 of FIG. 3A. The base portion 302 b has a surface 302 a. In one embodiment, the thickness of the base portion 302 b is not less than 100 μm. In another embodiment, the thickness of the base portion 302 b is not more than 300 μm. The plurality of protrusions 302 c is arranged in a two-dimensional array on the surface 302 a of the base portion 302 b, and the arrangement of the plurality of protrusions 302 c can be regular or irregular. Each protrusion 302 c includes a first portion 302 d and a second portion 302 e on the first portion 302 d. The first portion 302 d is integrated with the base portion 302 b and the second portion 302 e is formed on the first portion 302 d. Specifically, the first portion 302 d includes a first material that is the same as the material of the base portion 302 b. The second portion 302 e includes a second material that is different from the first material. In the embodiment, the refractive index of the second material is smaller than the refractive index of the first material. Specifically, the refractive index of the second material of the second portion 302 e at the dominant wavelength is smaller than the refractive index of the first material of the first portion 302 d at the dominant wavelength. In one embodiment, in terms of the dominant wavelength, the difference between the refractive index of the second material of the second portion 302 e and the refractive index of the first material of the first portion 302 d is greater than 0.1; in another embodiment, the difference is greater than 0.15; and still in another embodiment, the difference is between 0.15 and 0.4 (both inclusive). The three-dimensional shape of the protrusion 302 c includes a dome, a cone, or a pyramid. The cone includes a truncated cone and the pyramid includes a polygonal pyramid or a truncated pyramid. In the embodiment, the three-dimensional shape of the protrusion 302 c is a cone and the protrusion 302 c is substantially triangular in the cross-sectional view. The material of the second portion 302 e can be silicon dioxide (SiO₂). The material of the first portion 302 d and the base portion 302 b can be sapphire, and the surface 302 a is c-plane which is suitable for epitaxial growth. The buffer layer 304 is conformably formed on the plurality of protrusion 302 c and the surface 302 a. In one embodiment, the buffer layer 304 comprises aluminum nitride (AlN). In one embodiment, the thickness of the buffer layer 304 is greater than 5 nm; in another embodiment, the thickness is not more than 50 nm; and still in another embodiment, the thickness is between 10 nm and 30 nm (both inclusive). If the thickness of the buffer layer 304 is less than 5 nm, the defect density of the epitaxial layers (e.g. the first semiconductor layer 306) subsequently grown thereon becomes high and the epitaxial quality of the light-emitting device is disrupted. If the thickness of the buffer layer 304 is more than 50 nm, for example, an AlN buffer layer with a thickness of more than 50 nm, the dominant wavelengths of the plurality of light-emitting devices epitaxially grown on the same wafer are inconsistent with each other. As shown in FIG. 3B, an included angle θ is between the surface 302 a and the side surface of one protrusion 302 c or each of the protrusions 302 c. In one embodiment, θ is not greater than 65 degrees; in another embodiment, θ is not more than 55 degrees; and still in another embodiment, θ is between 30 and 65 degrees (both inclusive). In one embodiment, two included angles θ are between the surface 302 a and the side surface of one protrusion 302 c or each protrusion 302 c and θ are not greater than 65 degrees; in another embodiment, θ are not greater than 55 degrees; and still in another embodiment, θ are between 30 and 55 degrees (both inclusive). In one embodiment, the two included angles θ of one protrusion 302 c or the two included angles θ of each protrusion 302 c have the same degrees or different degrees. Each of the protrusions 302 c has a height H and a bottom width W. In one embodiment, the ratio of the height H to the bottom width W is greater than 0 and not more than 0.5. In one embodiment, the ratio of the height (H1) of the first portion 302 d of one or of each protrusions 302 c to the height (H) of the protrusion 302 c ranges between 1% and 30% (both inclusive). In another embodiment, the ratio of the height (H1) of the first portion 302 d of one or of each protrusion 302 c to the height (H) of the protrusion 302 c ranges between 10% and 30% (both inclusive). As shown in the figures, the arrangement of the protrusions 302 c has a period P. In one embodiment, the protrusion 302 c has a vertex, and the vertex is the part of the protrusion 302 c which is closest to the light emitting structure 308 along the direction T1. The period P is defined as the distance between the vertices of two adjacent protrusions 302 c. In this embodiment, the cross-section of the protrusion 302 c is substantially triangular, and the period P is between 1 μm and 3 μm (both inclusive). In one embodiment, the height H=1.2 μm±10%; the bottom width W=2.6 μm±10%; the period P=3.0 μm±10%. In another embodiment, the height H=0.9 μm±10%; the width W=1.6 μm±10%; the period P=1.8 μm±10%. In another embodiment, H=1 μm±10%; W=1.5 μm±10%; P=1.8 μm 10%. In another embodiment, the height H=1.2 μm±10%, the width W=2.6 μm±10%; and the period P=3.0 μm±10%. In one embodiment, XRD analysis is used to investigate the material characteristics of the epitaxy layers of the light-emitting device 300. The light-emitting device 300 has a FWHM value of smaller than 250 arcsec in accordance with a (102) XRD rocking curve. In one embodiment, the FWHM is not smaller than 100 arcsec.

As shown in FIG. 3A, a method for fabricating a light-emitting device 300 in accordance with an embodiment of the present application includes: providing a substrate structure 302 including the base portion 302 b and the plurality of protrusions 302 c formed thereon, which includes providing a substrate (not shown) having a top surface (not shown), and then performing a patterning step to form the base portion 302 b and the plurality of protrusions 302 c space apart from each other on the base portion 302 b. The patterning step includes forming a precursor layer (not shown) on the top surface of the substrate by, for example, physical vapor deposition (PVD), and then removing a portion of the precursor layer and a portion of the substrate from the top surface. The method of removing the portion of the precursor layer and the portion of the substrate includes dry etching or wet etching. The portion of the precursor layer and the portion of the substrate are removed to form the based portion 302 b with the surface 302 a and the other portions of the precursor layer and the substrate kept on base portion 302 b form the plurality of protrusions 302 c separated from each other. In the embodiment, the protrusion is substantially triangular in a cross-sectional view. The plurality of protrusions 302 c is arranged in a two-dimensional array on the surface 302 a of the base portion 302 b, and the arrangement of the plurality of protrusions 302 c can be regular or irregular. The method for fabricating the light-emitting device 300 in accordance with an embodiment of the present application includes forming the buffer layer 304 on the surface 302 a of the base portion 302 b and covering the protrusions 302 c. The buffer layer 304 comprises aluminum nitride (AlN). The method of forming a buffer layer 304 includes physical vapor deposition (PVD). The method of fabricating the light-emitting device 300 further includes forming the first semiconductor layer 306, the light-emitting structure 308, and the second semiconductor layer 310 by epitaxial growth such as metal organic chemical vapor deposition (MOCVD). The method of pitaxial growth includes, but is not limited to, MOCVD, hydride vapor phase epitaxial (HVPE), or liquid-phase epitaxy (LPE).

As shown in FIG. 3A, the light-emitting device 300 includes a cap layer 340 between the buffer layer 304 and the first semiconductor layer 306. The cap layer 340 has a thickness T that is greater than the thickness of the buffer layer 304. The cap layer 340 comprises III-V compound semiconductor having an energy gap smaller than that of the material of the buffer layer 304. In an embodiment, the cap layer 340 comprises gallium nitride (GaN). Specifically, the cap layer 340 covers the buffer layer 304, and a portion of the cap layer 340 is located in the recess 311. The cap layer 340 includes a top surface 3401 opposite to the buffer layer 304, and the thickness T of the cap layer 340 refers to the shortest distance between the first portion 3042 of and the top surface 3401. In one embodiment, the cap layer 340 has a thickness T greater than 1 μm; in another embodiment, the thickness T of the cap layer 340 is not more than 3.5 μm; and still in another embodiment, the thickness T of the cap layer 340 is between 1 μm and 2 μm. In an embodiment, the cap layer 340 includes impurities that are not intentionally doped. Specifically, the impurity concentration of the cap layer 340 is not more than 5×10¹⁷/cm³, and in one embodiment, not more than 1×10¹⁷/cm³. In this embodiment, since the plurality of protrusions 302 c is formed on the surface 302 a of the substrate structure 302 and the height of the protrusions 302 c is not more than 1.5 the cap layer 340 of the light-emitting device 300 is thinner than that of the conventional light-emitting device, yet the light-emitting device 300 has substantially the same performance as the conventional light-emitting device. Therefore, the light-emitting device 300 in the embodiment has an advantage of being small in size.

FIGS. 4A and 4B show top views of different embodiments of the substrate structure 102 of the light-emitting device in the present application. As shown in FIG. 4A, the surface 102 a of the base portion 102 b of the substrate structure 102 has a plurality of protrusions 102 c having circular contours in the top view, and the plurality of protrusions 102 c can be arranged in a hexagonal repeating pattern. As shown in FIG. 4B, a plurality of protrusions 102 c are formed on the surface 102 a of the base portion 102 b of the substrate structure 102. Each protrusion 102 c has a triangular contour in the top view, and one of the edges of each triangular contour can be a curve. The plurality of protrusions 102 c can be arranged in a hexagonal repeating pattern.

FIGS. 5A and 5B show cross-sectional views of different embodiments of the protrusion of the substrate structure of the light-emitting device in the present application. As shown in FIG. 5A, the protrusion 102 c is substantially a trapezoid. Specifically, the protrusion 102 c has an upper plane P1 and a lower plane P2 opposite to the upper plane P1, and the lower plane P2 is closer to the surface 102 a of the base portion 102 b than the upper plane P1. In one embodiment, the ratio of an area of the upper plane P1 to an area of the lower plane P2 is greater than zero and not more than 0.3. In one embodiment, as shown in FIG. 4B and FIG. 5A, the three-dimensional shape of the protrusion 102 c is a truncated triangular pyramid. In the top view, the triangular contour of the lower plane P2 surrounds the triangular contour of the upper plane P1. Three inclined surface S are respectively formed between each edge of the triangle of P2 and each edge of the triangle of the upper plane P1. In the embodiment, the period P is defined as the shortest distance between the centers of the planes P1 of the two adjacent protrusions 102 c.

As shown in FIG. 5B, the protrusion 102 c includes an arc 1021 which protrudes outward, and two ends of the arc 1021 are connected to form a pseudo chord 1022. The protrusion 102 c includes a top portion 201 connecting to the arc 1021. In the embodiment, the maximum distance B between the arc 1021 and the chord 1022 is greater than 0 μm, and in another embodiment, not greater than 0.5 μm. The width D of the top portion 201 of the protrusion 102 c is the maximum distance between any two points on the peripheral of the top portion 201. In one embodiment, the width D of the top 201 is zero. In another embodiment, the width D of the top 201 is greater than zero. θ is an angle between the surface 102 a and the chord 1022. In the present embodiment, the height H is greater than 0 μm and not more than 1.5 μm.

It will be apparent to those having ordinary skill in the art that various modifications and variations can be made to the devices in accordance with the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure covers modifications and variations of this disclosure provided they fall within the scope of the following claims and their equivalents. 

What is claimed is:
 1. A light-emitting device, comprising: a substrate structure, comprising a base portion having a surface and a plurality of protrusions regularly formed on the base portion; a buffer layer covering the plurality of protrusions and the surface; and III-V compound semiconductor layers formed on the buffer layer; wherein one of the plurality of protrusions comprises a first portion and a second portion formed on the first portion and the first portion is integrated with the base portion; and wherein the base portion comprises a first material and the first portion comprises the first material.
 2. The light-emitting device according to claim 1, wherein the second portion comprises a second material which is different from the first material and a refractive index of the second material is smaller than a refractive index of the first material.
 3. The light-emitting device according to claim 1, wherein the first portion of one of the plurality of protrusions has a height H1 and the one of the plurality of protrusions has a height H; wherein a ratio of H1 to H is between 1% and 30% both inclusive.
 4. The light-emitting device according to claim 2, wherein the buffer layer comprises a third material which is different from the second material.
 5. The light-emitting device according to claim 4, wherein the buffer layer comprises aluminum nitride.
 6. The light-emitting device according to claim 2, wherein the second material comprises SiO₂.
 7. The light-emitting device according to claim 1, wherein the first material comprises sapphire and the surface is c-plane of the sapphire.
 8. The light-emitting device according to claim 1, wherein the first portion comprises a planar top and the second portion is formed on and contacts the planar top.
 9. The light-emitting device according to claim 1, wherein an arrangement of the plurality of protrusions has a period and the period is between 1 μm and 3 μm both inclusive.
 10. The light-emitting device according to claim 1, wherein in a cross-sectional view, an included angle between a side surface of one of the plurality of protrusions and the surface of the base portion is less than 65 degrees.
 11. The light-emitting device according to claim 1, wherein a thickness of the buffer layer is greater than 5 nm and not more than 50 nm.
 12. The light-emitting device according to claim 1, wherein a three-dimensional shape of one of the plurality of protrusions comprises a dome, a cone or a pyramid.
 13. The light-emitting device according to claim 1, wherein in a cross-sectional view, one of the plurality of protrusions comprises an arc which protrudes outward, and two ends of the arc are connected to form a pseudo chord; and wherein a maximum distance between the arc and the pseudo chord is greater than 0 μm and not greater than 0.5 μm.
 14. A method of manufacturing a light-emitting device, comprising: providing a substrate structure comprising a base portion and wherein the base portion comprises a surface; performing a patterning step to form a plurality of protrusions, wherein the plurality of protrusions are arranged on the surface of the base portion; forming a buffer layer on the surface of the base portion by physical vapor deposition, wherein the buffer layer covers the protrusions; and forming III-V compound semiconductor layers on the buffer layer; wherein one of the plurality of protrusions has a height not greater than 1.5 μm; and wherein the light-emitting device has a full width at half maximum (FWHM) of smaller than 250 arcsec in accordance with a (102) XRD rocking curve.
 15. The method according to claim 13, wherein the patterning step comprises forming a precursor layer on the surface and removing a portion of the precursor layer to form the plurality of protrusions.
 16. The method according to claim 15, wherein the base portion comprises a first material and the precursor layer comprises a second material which is different from the first material.
 17. The method according to claim 14, wherein the buffer layer is conformally formed on the plurality of protrusion and the surface.
 18. The method according to claim 14, wherein in a cross-sectional view, an included angle between a side surface of one of the plurality of protrusions and the surface of the base portion is less than 65 degrees.
 19. The method according to claim 14, wherein a method of forming the III-V compound semiconductor layers is different from physical vapor deposition. 